This invention relates to an activated gas generator which applies a microwave electric energy to a gas under reduced pressure.
Recently developed is an apparatus which subjects gas under reduced pressure to the discharge by microwave energy having a frequency ranging from 300 MHz to 30 GHz (hereinafter referred to as "microwaves") to activate said gas and applies the activated gas in the etching of a silicon wafer, the incineration of a photoresist, and the improvement of the hydrophilic and adhesive property of the surface of plastics and metals.
FIG. 1 shows the arrangement of the prior art apparatus for activating a gas by microwave energy and treating various substances by the activated gas. Microwave energy produced by a microwave power generator 1 provided with a microwave tube used with, for example, a magnetron is conducted to a waveguide 2. Thereafter, microwave energy is sent forth to a microwave irradiation furnace 6 through an isolator 3, power monitor 4 for detecting reflected electric energy and a tuner for impedance matching unit 5. The irradiation furnace 6 contains a movable short-plunger 7 for impedance matching. A metal tube 8 projects outward from the inner wall of the microwave irradiation furnace 6 in which a magnetic field is created. A gas pipe 9 penetrates the microwave irradiation furnace 6 and metal tube 8. That portion 10 of the gas pipe 9 where a gas is activated by microwave energy is formed of dielectric material. The gas pipe 9 is connected at one end to a raw gas tank 12 through a valve 11 and at the other end to a reaction chamber 13 for treating an object material. A vacuum pump 14 is connected to the reaction chamber 13 to evacuate the gas pipe 9. A gas introduced from the gas tank 12 under reduced pressure is activated in the microwave irradiation furnace 6 by microwave energy produced by a microwave power generator, and then brought into the reaction chamber 13.
With an activated gas generator constructed as described above, most of the microwave energy produced is used to activate a gas introduced. However, part of the microwaves leaks through an interstice between the metal tube 8 and gas pipe 9. To reduce said leakage to, for example, less than 1 mW/cm.sup.2, the metal tube 8 was formerly made sufficiently long to seal the glowing portion owing to gas discharge of the gas-activating region of the microwave irradiation furnace 6. Where oxygen plasma was produced by the microwave energy 10.8 KW having a frequency of, for example 2450 the metal tube 8 had to be made longer than 250 mm. Therefore, particularly that portion 10 of the gas pipe 9 which extended from the microwave irradiation furnace 6 down to the reaction chamber 13 was made longer in proportion to the whole length of the metal tube 8. Extension of the gas pipe is undersirable, because particles of activated gas are recombined, more prominently rendering the whole gas unactivated.
While a gas was not exposed to the discharge of microwave energy, said energy obviously did not leak from the metal tube 8. Yet when the gas has been activated by the discharge of microwave energy, then said energy prominently leaked. Particularly, the gas outlet section of the gas pipe 9 which was connected to the vacuum pump 14 was found to indicate a more noticeable leakage of microwave energy than the gas inlet section of the gas pipe 9. It is further knwon that a dielectric material constituting that portion of the gas pipe 9 which was enclosed in the microwave irradiation furnace 6 where gas under reduced pressure was activated by the discharge of microwave energy had sometimes its temperature raised over 300.degree. C.
A dielectric material such as quartz, alumina porcelain more increased in dielectric loss according as its temperature rose. Consequently, the dielectric material itself absorbed a larger amount of microwave energy, resulting in a further increase in said temperature. Elevated temperature of the dielectric material not only gave rise to difficulties in handling a gas-activating apparatus, but also caused the dielectric material itself to be partly etched by some kind of activated gas. Heat indicated by the above-mentioned higher temperature of the dielectric material than 300.degree. C. was transmitted to the reaction chamber 13, causing the whole activated gas generator to be highly heated. Therefore, full consideration had to be given to the heat-resistant property of that portion of the reaction chamber 13 which contacted the dielectric material of the gas pipe 9. Consequently, the gas pipe 9 and reaction chamber 13 had to be integrally constructed of the same material, raising great problems in respect of the handling and production cost of an activated gas generator.
Hitherto, therefore, cooling air was introduced into the microwave irradiation furnace 6 at one end to cool the dielectric tube. Or the gas pipe 9 was cooled by forcefully blowing cooling air to the outside.
With the prior art gas-activating apparatus, therefore, means for preventing the leakage of microwaves was provided separately from means for cooling the dielectric tube which tended to be highly heated. Yet neither of these means provided satisfactorily effective.